Small switch mode AC/DC power supplies or adapters are now starting to replace 50/60 Hz transformer “linear” adapters. They are lighter, smaller, and are cost competitive with the “linear” adapters. One of the main areas of use for these adapters is as battery chargers for GSM and other types of cellular telephones. With the standby power consumption of these telephones getting continuously lower, the battery sizes for these telephones are also getting smaller. A two-watt adapter charger is adequate for charging such a battery in only a few hours.
Because of the very low cost of the linear chargers, only the lowest cost “switching” topology is capable of competing in terms of cost. This topology is usually a self-oscillating fly back converter using a high voltage bipolar transistor as a main switch. FIG. 18 shows a functional diagram of a typical self-oscillating converter.
The transformer is both a costly and physically large part of a power supply. The large size is due in part to the safety creepage and clearances required between the primary and secondary windings of the transformer. Creepage and clearance distances are a significant factor in determining the physical size of the transformer. While triple insulation on the secondary wire can be used to keep the size of the transformer small, the use of triple insulation is expensive. The concentric winding arrangement of the transformer's windings also results in high common mode EMI, which usually requires an electrostatic shield winding and a common mode filter capacitor.
Embodiments of the invention address these and other problems.